1. Field of the Invention
The present invention relates to a mobile node, a method or handover and a computer program.
A mobile communication device can be understood as a device provided with appropriate communication and control capabilities for enabling use thereof for communication with others parties. The communication may comprise, for example, communication of voice, electronic mail (email), text messages, data, multimedia and so on. A communication device typically enables a user of the device to receive and transmit communications via a communication system and can thus be used for accessing various applications.
A communication system is a facility which supports communication between two or more entities such as mobile communication devices, network entities and other nodes. A communication system may be provided by one or more interconnected networks. One or more gateway nodes may be provided for interconnecting various networks of the system. For example, a gateway node may be provided between an access network and other communication networks, for example a core network and/or a data network.
An appropriate access network allows the communication devices to access the wider communication system. Access to the wider communications system may be provided by means of a fixed line or wireless communication interface, or a combination of these. Communication systems provide wireless access that enable at least some mobility for the users thereof. Examples of these include wireless communication systems where the access is provided by means of an arrangement of cellular access networks. Other examples of wireless access technologies include different wireless local area networks (WLANs) and satellite based communication systems.
A wireless access network typically operates in accordance with a wireless standard and/or with a set of specifications which set out what the various elements of the system are permitted to do and how that should be achieved. For example, the standard or specification may define if the user, or more precisely the mobile communication device, is provided with a circuit switched bearer or a packet switched bearer, or both. Communication protocols and/or parameters which should be used for the connection are also typically defined. For example, the manner in which communication should be implemented between the mobile communication device and the elements of the networks and their functions and responsibilities are typically defined by a predefined communication protocol.
The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. TCP provides reliable, in-order delivery of a stream of bytes, making it suitable for applications such as file transfer and e-mail.
However, TCP performance has not been engineered for mobility events in multi-radio mobile hosts. Traditionally, TCP has applied a set of congestion control algorithms (slow-start, congestion avoidance, fast retransmit, fast recovery) to probe the currently available bandwidth on the established connection path. These algorithms need several round-trip times to find the correct transmission rate (i.e., congestion window), and adapt to sudden changes, for example due to mobile hand-off or handover, slowly. Handover is when a mobile node hands off from one access node to another. This handoff may be between access nodes of different networks.
TCP is implemented as a separate component as a part of communication/networking subsystem in an operating system, and is therefore often unaware of the mobility events occurring due to the nature of a multi-radio communication, at a lower layer. Multi-radio devices are able to deal with two or more different radio access technologies simultaneously.
In a heterogeneous networking environment, the Mobile Node (MN) or user equipment often encounters instant changes in link performance and coverage. However such abrupt changes in link quality or coverage either interrupts or terminates an ongoing TCP connection and eventually deteriorates the performance of the ongoing application flows, in addition to delaying the registration process for mobility protocols (for example, registering to Rendez-vous Server (RVS) for Host Identity Protocol (HIP), or binding updates to a Home Agent (HA) for Mobile Internet Protocol (MIP)). Furthermore there is no classical TCP stack which can handle connections efficiently on these paths with changing heterogeneous characteristics.
Additionally different applications have different networks and TCP requirements. TELNET values short response time (delay sensitivity), but the File Transfer Protocol values high throughput (delay tolerant). Moreover applications run on a mobile node such as a phone or small PDA (personal data assistant) have limited power. Generally TCP may avoid unnecessary retransmission to save energy. TELNET (TELecommunication NETwork) is a network protocol used on the Internet or local area network (LAN) connections.
A TCP window defines the amount of outstanding (unacknowledged by the recipient) data a sender can send on a particular connection before the sender gets an acknowledgment back from the receiver that the receiver has received some of that data.
It has been proposed to adjust a TCP's receive window to suppress the TCP sender from sending further data, and to adjust the receive window to be better suitable for a slow modem link. However, this has targeted environments with static and fixed setup, and has not been applied in a dynamic network environment with different types of applications and multiple available networks with terminal mobility.
It has also been proposed to adjust a TCP's slow-start threshold based on explicit indication of available bandwidth on the path. This requires modifications to the TCP sender side that is often a fixed server and therefore inaccessible to the mobile terminal vendors.
The paper titled “Using Quick-Start to Improve TCP Performance with Vertical Hand-offs” at http://csd12.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp /proceedings/lcn/2006/0418/00/0418toc.xml&DOI=10.1109/LCN.2006.322197] discusses the use of slow-start threshold after vertical handoff, not receiver window.
“Experiences with Heterogeneous Wireless Networks, Unveiling the Challenges” by Vidales et al. describes setting the receiver window to zero during the handoff to suspend the sender and avoid packet losses. This is available from:    [http://www.cl.cam.ac.uk/research/dtg/publications/public/pav25/HetNets04-Vidales.pdf].
Reference is made to “TCP auto-tuning zoo” which describes receive buffer auto tuning based on bandwidth-delay product, but this does not discuss the vertical handoff case: This document is available from [http://www.csm.ornl.gov/˜dunigan/net100/auto.html]
“Multi-Layer Protocol Tracing in a GPRS Network” by Gurtove et al (available from [http://www.cs.helsinki.fi/u/gurtov/papers/vtc02.pdf]), suggests that the buffer of all links should be configured with maximum bandwidth delay product. However this relies on the operator and this might be difficult to justify when there is a link with low bandwidth-delay product, and may cause excess queuing.
Reference is made to the IETF (Internet Engineering Task Force) Draft Specification Mobility Services Transport: Problem Statement draft-ietf-mipshop-mis-ps-03 which is available from http://www.ietf.org/internet-drafts/draft-ietf-mipshop-mis-ps-03.txt. This draft provides the Mobility service transport protocol (MSTP) for the Media Independent Handover (MIH) protocol (MIH is an IEEE solution within 802.21 TG), that can operate over network layer. The requirements for MSTP include discovery, security, low latency, reliability etc. This draft indicates that handover support in heterogeneous wireless environments requires the functional components located within in a mobile terminal or in the network to exchange the handover or mobility relevant information and eventually to take decisions upon this information exchange. The traditional host-based handover solution could be complemented with more sophisticated network-centric solutions.
US20060291435 adjusts TCP parameters/windows according to used wireless bearer with a preconfigured window size for each bearer.
Reference is made to the IETF (Internet Engineering Task Force) Draft Specification Lightweight Mobility Detection and Response (LMDR) Algorithm for TCP draft-swami-tcp-lmdr-07 which is available from http://www1.tools.ietf.org/html/draft-swami-tcp-lmdr-07 which deals with TCP.
Another approach is for wireless link endpoints to choke TCP senders during handoffs, by transparently closing the receiver's advertised window (See “TCP Performance Issues over Wireless Links” IEEE Communications Magazine, Vol 39, Number 4, 2001, pages 52-58).
US20030219034 describes reducing TCP window size if link quality deteriorates.
It is an aim of certain embodiments of the present invention to solve one or more of the aforementioned problems.